Suspensions of lime particles, often called lime slurries, are used in a variety of diverse applications. For example, lime slurries are commonly used in a water-softening process called “lime softening,” where lime causes ‘hardness’ ions to precipitate from and settle out of solution thus softening the water. In wastewater and process water applications it can be desirable to use lime slurries (instead of conventional caustic NaOH solutions) to adjust the pH of, or to neutralize, acidic solutions on a commercial, municipal or industrial scale. One example of this is in sewage and wastewater treatment, where caustic solutions of sodium hydroxide are commonly used to treat large amounts of water that have an acidic pH, or to increase the pH of the water to kill bacteria, microbes and other organisms. To neutralize acidic solutions of both raw and treated water, caustic solutions are often used.
High-concentration caustic solutions (e.g. 50% NaOH, balance water) are typically used for many of these applications due to the difficulty in handling lime and its traditional slurries, described in summary below. Caustic solutions are highly dangerous and must be carefully controlled to ensure that humans do not come into contact with the solutions, which can cause severe burns. In addition, the solid NaOH particles that often are used to make caustic solutions by dissolution in water also present serious health concerns, and their use, shipment and storage must be carefully controlled, and are subject to numerous safety regulations. Lime is considerably less expensive than NaOH and also preferable from a health-safety standpoint, but handling problems associated with lime typically overshadow its use.
Lime slurries are sometimes used as a substitute for caustic solutions to raise or neutralize the pH of commercial-scale acidic solutions such as treated wastewater. Unlike NaOH solution, these slurries are not true solutions but instead are suspensions of solid particles of hydrated lime in water. To make a lime slurry, particles of quicklime (CaO) are added to a water carrier, wherein the quicklime particles are hydrolyzed to produce particles of hydrated lime (Ca(OH)2). The hydrolysis reaction is extremely exothermic, producing a large amount of heat and raising the temperature of the slurry. For example, to make a ˜30 to ˜40 weight percent Ca(OH)2 suspension in ˜24,000 gallons of water, sufficient heat can be generated to raise the temperature of the slurry to 225° F. or greater before the reaction is completed and the mixture begins to cool.
To approach the neutralization power of conventional caustic solutions, very-high solids lime slurries must be used, for example at or exceeding 30% hydrated lime solids by weight. But using such high-content lime solids is problematic because the resulting viscosity can render the slurries impractical or un-useful from a materials-handling standpoint. Gypsum can be used to reduce the viscosity of a high-solids content lime slurry, but conventionally the addition of gypsum has been known to cause large agglomerations of lime particles to form, which tend to come out of suspension more readily and may block pipes or other materials-handling equipment.
Another method to moderate the viscosity of a high-solids content hydrated lime suspension is to incorporate a polymeric dispersing agent. For example, certain polyacrylic acids have been used as dispersing agents to moderate the viscosity of such a high-solids suspension. However, the use of such dispersing agents to date has been limited because they become unstable and ineffective if exposed to temperatures above the boiling point of water. Accordingly, when such polymeric dispersing agents have been used, it has been necessary either to perform a time-consuming stepwise CaO-addition procedure—wherein quicklime is introduced into the water in relatively small, successive increments to ensure the slurry temperature never approaches the boiling point of water (which would inactivate the dissolved polymeric dispersant), or to use hydrated lime from the outset and mix it into the suspension. This latter method will avoid entirely the heat of reaction associated with hydrolysis of quicklime because the solids are already in the hydrated form when introduced into the suspension. However, this requires the additional step of pre-hydrating the quicklime before adding it to the slurry mixture, or otherwise purchasing hydrated lime for use in the suspension instead of quicklime, which may result in greater expense. In addition, hydrated lime is over 30% heavier than the corresponding amount of quicklime, which will result in significantly larger transportation and handling costs prior to mixing it into a slurry.
It is desirable to employ a hydrated-lime slurry having a dispersant therein that effectively moderates the viscosity to permit a usable slurry having at least 30 weight percent hydrated lime, and which is not inactivated at temperatures experienced as a result of the hydrolysis of quicklime to produce hydrated lime in the slurry.